A Potential Role for Human Cohesin in Mitotic Spindle Aster Assembly

Abstract

The cohesin multiprotein complex containing SMC1, SMC3, Scc3 (SA), and Scc1 (Rad21) is required for sister chromatid cohesion in eukaryotes. Although metazoan cohesin associates with chromosomes and was shown to function in the establishment of sister chromatid cohesion during interphase, the majority of cohesin was found to be off chromosomes and reside in the cytoplasm in metaphase. Despite its dissociation from chromosomes, however, microinjection of an antibody against human SMC1 led to disorganization of the metaphase plate and cell cycle arrest, indicating that human cohesin still plays an important role in metaphase. To address the mitotic function of human cohesin, the subcellular localization of cohesin components was reexamined in human cells. Interestingly, we found that cohesin localizes to the spindle poles during mitosis and interacts with NuMA, a spindle pole-associated factor required for mitotic spindle organization. The interaction with NuMA persists during interphase. Similar to NuMA, a significant amount of cohesin was found to associate with the nuclear matrix. Furthermore, in the absence of cohesin, mitotic spindle asters failed to form in vitro. Our results raise the intriguing possibility that in addition to its well demonstrated function in sister chromatid cohesion, cohesin may be involved in spindle assembly during mitosis.Proper chromosome segregation in mitosis is essential for the maintenance of chromosome integrity. Sister chromatid cohesion, which ensures the pairing of the two sister chromatids, is a prerequisite for proper chromosome segregation at anaphase. Cohesin is a multiprotein complex required for sister chromatid cohesion in eukaryotes. Cohesin was first characterized in Saccharomyces cerevisiae, which is composed of two structural maintenance of chromosome (SMC) 1 family proteins, Smc1p and Smc3p, as well as the non-SMC components Scc1 and Scc3p (Refs.1-3 and see review in Ref. 4). A similar complex was identified in Schizosaccharomyces pombe, Xenopus, and humans (5-7). In S. cerevisiae, cohesin functions in both the establishment and maintenance of sister chromatid cohesion from S phase to metaphase (1, 2). Consistently, cohesin in S. cerevisiae stays associated with chromosomes from the late G 1 phase until the onset of anaphase (3). Scc1p is a homolog of Rad21 identified earlier in S. pombe (8). Proteolytic cleavage of Scc1p by the cysteine protease separin, which is indirectly promoted by the anaphase-promoting complex at the end of metaphase, is responsible for the destruction of cohesin and the initiation of sister chromatid segregation in anaphase (9). A similar observation was made in S. pombe, although S. pombe cohesin stays associated with chromosomes throughout the cell cycle (7). Studies of S. cerevisiae identified the in vivo binding sites of cohesin on the chromosome arms at estimated intervals of 13 kb with the highest concentration at the centromeric regions (10 -12). Similar centromeric clustering of cohesin was also observed in S. pombe (7). In metazoa...

An ATM- and ATR-dependent checkpoint inactivates spindle assembly by targeting CEP63

“…Interestingly, cytoplasmic SMC1 protein of the cohesin complex, was found to localise to centrosomes (Gregson et al, 2001;Guan et al, 2008;Kong et al, 2009). In vertebrates a cohesin function was shown to be necessary in aster assembly and required in microtubule spindle organisation (Kong et al, 2009).…”

“…In vertebrates a cohesin function was shown to be necessary in aster assembly and required in microtubule spindle organisation (Kong et al, 2009). Cohesin has been shown to interact with NuMa prior to mitosis, it is hypothesized that NuMa transports cohesin to spindle poles in a dynein dependent manner (Gregson et al, 2001;Kong et al, 2009;Merdes et al, 2000).…”

Cohesin Associates with Spindle Poles in a Mitosis-specific Manner and Functions in Spindle Assembly in Vertebrate Cells

“…Because this particular antibody detects hSMC1 in the cytoplasm, but not at the pairing domain of metaphase chromosomes (likely due to steric hindrance), the notion was raised that cytoplasmic cohesin may play an important role in mitosis. Subsequently, we found that a subpopulation of cytoplasmic cohesin localizes to the mitotic spindle poles and interacts with the nuclear mitotic apparatus protein (NuMA) (Gregson et al, 2001). NuMA is a factor that localizes to the spindle poles in a mitosis-specific manner and plays an important role in mitotic spindle assembly at the spindle poles (Gaglio et al, 1995;Merdes et al, 1996).…”

“…Wild-type, CENP-H or Scc1/Rad21 conditional knockout DT40 cells were cultured as described previously (Fukagawa et al, 2001(Fukagawa et al, , 2004Sonoda et al, 2001). HeLa cells were synchronized to S phase by using double-thymidine block and to M phase by an additional nocodazole treatment as described previously (Schmiesing et al, 2000;Gregson et al, 2001). …”

“…HeLa cells were synchronized to S phase by using double-thymidine block and to M phase by an additional nocodazole treatment as described previously (Schmiesing et al, 2000;Gregson et al, 2001).…”